In a multiple-ratio transmission having a hydrokinetic torque converter and multiple-ratio gearing, engine torque is delivered to the impeller of the converter as the turbine of the converter drives the input element of the multiple-ratio gearing. Shifts between ratios is controlled by a shift valve system. When the vehicle is coasting with the engine throttle closed, the shift valve system will effect coasting downshifts. At that time, the vehicle is traveling at a relatively low speed and the engine is at or near an idle state.
During a coasting downshift from a ratio in which a friction clutch is engaged to frictionally connect two elements of the gearing to a ratio in which a reaction element is held by a friction brake, the shift is characterized as a synchronous shift. That is, the clutch must be released in synchronism with the application of the brake. In such instances, the capacity of the clutch is in excess of the capacity that would be required during a coasting downshift. The brake, which is operated by a brake servo, has a higher pressure requirement.
The pressure requirement of the brake and the capacity of the clutch are designed to meet the requirements of the driveline during normal torque delivery. This, of course, results in excess capacity for the clutch during a coasting downshift when the driving torque is at minimal levels.
Such a synchronous downshift may result in a torque reversal in the driveline, which results in an audible torque disturbance as the powertrain experiences a transition from an operating mode in which the engine drives the wheels of the vehicle before the shift to an operating mode in which the wheels drive the engine after the shift. As the so-called lash in the transmission is traversed at the transition point, the shift feel may be objectionable, particularly when the torque disturbance is audible during a coasting downshift.
In those special instances in which the engine speed is decaying normally on a coasting downshift as the operator closes the engine throttle, the torque gradient from the torque delivery state to a coasting torque state will be slight, and the lash in the driveline will not result in an audible torque disturbance. On the other hand, if the oncoming friction brake "pulls" the output torque through the transition point as the torque direction is reversed, the torque gradient will be steep and the downshift quality will be unacceptable.
Transmissions typically are calibrated for optimal downshift points on a coasting downshift when the engine is operating at its normal operating temperature. During operation of the engine at low engine temperature following a cold start, however, the engine control strategy usually requires an increase in engine speed. This is done by increasing the duty cycle of a duty cycle controlled bypass air valve. The idle air bypass can be controlled in other ways, of course, other than by using a duty cycle controlled bypass air valve.
If the coasting downshift should occur while the engine is operating with an increased speed following a cold start, a synchronous coasting downshift typically is accompanied by a torque reversal at the operating mode transition point that may degrade the downshift quality.